1. Field of the Invention
The present invention relates to a semiconductor wafer with an uneven distribution of crystal lattice defects, and to a process for producing this wafer.
2. The Prior Art
Silicon crystals, in particular for the production of semiconductor wafers, are preferably obtained by pulling a seed crystal from a silicon melt, which is generally provided inside a quartz glass crucible. This so-called Czochralski crucible-pulling process is described in detail in, for example, W. Zulehner and D. Huber, Czochralski-Grown Silicon, Crystals 8, Springer Verlag Berlin-Heidelberg 1982.
Due to the reaction of the quartz glass crucible with the molten silicon during the crucible-pulling process, oxygen is included as the dominant impurity in the growing silicon crystal. The concentration of oxygen is usually so high that, after the crystal has cooled, it is in supersaturated form. In subsequent heat treatments, the oxygen is deposited in the form of oxygen precipitates. These precipitates have both advantages and disadvantages. The so-called gettering properties of the oxygen precipitates are an advantage.
This is understood to mean that, for example, metallic impurities in the semiconductor wafer are bonded to the oxygen precipitates. Thus, they are removed from the layer which is close to the surface and is relevant for components. A drawback is that oxygen precipitates in the layer which is close to the surface. This is relevant for components and will interfere with the function of the components which are manufactured on the semiconductor wafer. Consequently, it is desired for a precipitate-free zone, PFZ, also known as a denuded zone, DZ, to be formed in the vicinity of the surface. It is also desired for a high concentration of precipitates to be formed in the interior of the semiconductor wafer, known as the bulk.
The prior art, for example in xe2x80x9cOxygen in Siliconxe2x80x9d F. Shimura, Semiconductors and Semimaterials Vol. 42, Academic Press, San Diego, 1994, has disclosed how the outdiffusion of the oxygen near the surface is achieved in a heat treatment at temperatures of preferably over 1100xc2x0 C. As a result of this outdiffusion, the concentration of oxygen in the layer close to the surface falls so far that there is no longer any precipitation, and consequently a PFZ is generated. This heat treatment was in most cases directly integrated into the processes for producing components. In modern processes, however, these high temperatures are no longer used, and consequently the required outdiffusion is brought about by an additional heat-treatment step.
The oxygen precipitation, in particular in crucible-pulled semiconductor material, takes place substantially in two steps:
1) formation of nucleation centers for oxygen precipitates, so-called nuclei;
2) growth of these centers to form detectable oxygen precipitates.
During subsequent heat treatments, the size of these nuclei can be modified in such a way that those which have a larger radius than the so-called xe2x80x9ccritical radiusxe2x80x9d grow into oxygen precipitates. On the other hand, nuclei with a smaller radius break down (are dissolved). The growth of nuclei with a radius  greater than rc takes place at elevated temperature and is substantially limited by the diffusion of oxygen. A generally accepted model (cf., for example, Vanhellemont et al., J. Appl. Phys. 62, p. 3960, 1987) describes the critical radius rc as a function of the temperature, the oxygen supersaturation and the concentration of vacancies. Concentration is understood to mean particles per unit volume.
A high oxygen concentration and/or a high vacancy supersaturation simplifies or accelerates the precipitation of oxygen and leads to a higher concentration of precipitates. Furthermore, the concentration or size of the precipitates, in particular in semiconductor wafers, depends on heating and cooling rates during thermal furnace processes, in particular during the so-called RTA (rapid thermal annealing) processes. During these heat treatments, semiconductor wafers are heated to temperatures of up to 1300xc2x0 C. within a few seconds and are then cooled at rates of up to 300xc2x0 C./sec.
The oxygen concentration, the vacancy concentration, the interstitial concentration, the dopant concentration and the concentration of existing precipitation nuclei, such as for example carbon atoms, also influence the precipitation of oxygen.
WO 98/38675 has disclosed a semiconductor wafer with an uneven distribution of crystal lattice vacancies, which is obtained by means of a heat treatment. The maximum level of this vacancy profile generated in this way is situated in the bulk of the semiconductor wafer, and the profile decreases considerably toward the surfaces. During subsequent heat-treatment processes, in particular at 800xc2x0 C. for 3 h and 1000xc2x0 C. for 16 h, the oxygen precipitation follows this profile. The result is a PFZ without prior outdiffusion of the oxygen and oxygen precipitates in the bulk of the semiconductor wafer.
According to WO 98/38675, the concentration of oxygen precipitates is set by means of the concentration of vacancies, and the depth of the precipitates is set by means of the cooling rate following the heat treatment. A drawback of this semiconductor wafer is that the getter centers are limited to the bulk. Furthermore, very high BMD (bulk micro defect) concentrations lead to high leakage currents from integrated circuits when these circuits are located close to the layer relevant for the components. These leakage currents can be minimized if regions with very high BMD concentrations are produced as far away from the components as possible. Furthermore, it has been found that, in particular for applications in micromechanics, high BMD concentrations in the middle of the semiconductor wafer have an adverse effect on the selective etching behavior. This is because a high variation in etching removal rates is observed in the presence of the precipitates. Consequently, it is desired to limit the oxygen precipitates as far as possible to defined layers. It is also desired to keep the back-side part of the semiconductor wafer precipitate-free, for example for the production of micromechanical structures.
It is therefore an object of the present invention to provide a semiconductor wafer, and a process for producing the wafer, which serves as a basis for a semiconductor wafer with an improved internal gettering action.
This above object is achieved according to the invention by means of a semiconductor wafer having a front side 1, a back side 2, a top layer 3, a bottom layer 4, an upper inner layer 5 lying below the top layer 3, a lower inner layer 6 lying above the bottom layer 4, a central region 7 between the layers 5 and 6 and an uneven distribution of crystal lattice defects. The concentration of the defects exhibits a first maximum (max1) in the central region 7 and a second maximum (max2) in the bottom layer 4.
The defects are preferably vacancies which are converted into nucleation centers for oxygen precipitates during subsequent heat treatment processes, preferably at temperatures of from 300xc2x0 C. to 800xc2x0 C. According to the invention, the nucleation centers follow the profile of the vacancies. Preferably, the concentration of the defects increases from the front side 1 of the semiconductor wafer toward the central region 7, up to the first maximum (max1), and toward the bottom layer 4, up to the second maximum (max2).
Accordingly, the above object is also achieved by means of a semiconductor wafer with an uneven distribution of nucleation centers for oxygen precipitates.
In particular, the object is also achieved by means of a semiconductor wafer having a front side 1, a back side 2, a top layer 3, a bottom layer 4, an upper inner layer 5 lying below the top layer 3, a lower inner layer 6 lying above the bottom layer 4, a central region 7 between the layers 5 and 6 and an uneven distribution of nucleation centers for oxygen precipitates. The concentration of the nucleation centers exhibits a first maximum (max1) in the central region 7 and a second maximum (max2) in the bottom layer 4, and the concentration of the nucleation centers on the front side 1 and in the top layer 3 is so low that, in a subsequent heat treatment without outdiffusion of oxygen, a precipitate-free layer with a thickness of from 1 to 100 xcexcm is formed on the front side 1.
Surprisingly, it has been found that the nucleation centers are not mobile point defects, but rather immobile deposits. According to the invention, therefore, the oxygen precipitation exactly follows this profile during subsequent heat treatments, for example during a heat treatment for 3 h at 780xc2x0 C. and for 16 h at 1000xc2x0 C.
Due to the variation in concentration of the nucleation centers at increasing distance from the surface of the semiconductor wafer, the subsequent heat treatment processes thus result in a depth-dependent variation in the concentration of the oxygen precipitates.
Accordingly, a semiconductor wafer whose concentration of nucleation centers is very low on the front side 1 and in the top layer 3, increases toward the central region, to a first maximum (max1), and then rises again toward the back side 2, in order to reach a second maximum (max2) in the bottom layer 4, is preferred. The second maximum is greater than the first maximum. The front side (1) is that side of the wafer on which electronic devices will be produced.
However, the object is also achieved by means of a semiconductor wafer having a front side 1, a back side 2, a top layer 3, a bottom layer 4, an upper inner layer 5 lying below the top layer 3, a lower inner layer 6 lying above the bottom layer 4, a central region 7 between the layers 5 and 6 and an uneven distribution of crystal lattice defects, wherein the concentration of the defects exhibits a maximum (max1) in the upper inner layer 5.
The defects are preferably vacancies which are converted into nucleation centers for oxygen precipitates during subsequent heat treatment processes, preferably at temperatures of from 300xc2x0 C. to 800xc2x0 C. According to the invention, the nucleation centers follow the profile of the vacancies.
Accordingly, the object is also achieved by means of a semiconductor wafer with an uneven distribution of nucleation centers for oxygen precipitates.
In particular, the object is also achieved by means of a semiconductor wafer having a front side 1, a back side 2, a top layer 3, a bottom layer 4, an upper inner layer 5 lying below the top layer 3, a lower inner layer 6 lying above the bottom layer 4, a central region 7 between the layers 5 and 6 and an uneven distribution of nucleation centers for oxygen precipitates. The concentration of the nucleation centers exhibits a maximum (max1) in the upper inner layer 5. The concentration of the nucleation centers on the front side 1, the back side 2, in the top layer 3, the bottom layer 4 and the lower inner layer 6 is so low that, in a subsequent heat treatment without outdiffusion of oxygen, precipitate-free layers with a thickness of from 1 to 100 xcexcm are formed on the front side 1 and the back side 2.
According to the invention, therefore, the oxygen precipitation exactly follows this profile during subsequent heat treatments, for example during a heat treatment for 3 h at 780xc2x0 C. and for 16 h at 1000xc2x0 C.
The oxygen precipitates in particular in the upper inner layer 5 have proven advantageous, since the back-side region of precipitates remains clear and therefore offers ideal conditions for, for example, applications in micromechanics. Nevertheless, the precipitates formed in the layer 5 and in the central region 7 produce a good gettering action.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings which disclose embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.